This project addresses two major barriers to progress in allogeneic hematopoietic stem cell transplantation (HSCT): graft-vs-host disease (GvHD) and relapse. A novel strategy is proposed that challenges the prevailing practice of trying to curtail donor T cell alloreactivity after the fact, to one of controlling the afferent sensitization of lymphocyte effectors at the level of antigen-presentation by dendritic cells (DCs). DCs comprise a heterogeneous population of potent antigen-presenting cells, which initiate immunity and control its quality. We hypothesize that the undesirable complications of GvHD can be separated from the beneficial GvL effects exerted by allogeneic HSCT, based on differences in afferent sensitization of immune responses by distinct DC subtypes. This hypothesis will be addressed in the following specific aims: SPECIFIC AIM 1: Determine whether allogeneic interactions between human Langerhans-type DCs (LCs) and T cells in vitro are abrogated by inhibition of JAK2-STAT3 signaling, and whether the capacity of WTI mRNA- electroporated LCs remains intact to break tolerance against this self-differentiation tumor Ag by an IL15- dependent mechanism. SPECIFIC AIM 2: Compare and contrast the inhibition of JAK2-STAT3 in NK cells stimulated by monocyte-derived DCs (moDCs) via an IL12p70-dependent mechanism, with JAK2 inhibition of moDC-stimulated allogeneic T cells. SPECIFIC AIM 3: Evaluate the mechanism of JAK2 inhibition with respect to alloreactivity and tumor immunity in defined mouse transplant models. Successful inhibition of the JAK2-pSTAT3 pathway, which would block at least moDC-derived IL6 and IL23 cytokine effects on alloreactive lymphocytes, would lead to the use of an entirely new class of drugs for prevention and treatment of GvHD. This would maintain freedom from relapse (GvL) and infectious morbidity and mortality brought on by less selective immune suppression. The approach would also take full advantage of the potency of W T I mRNA-electroporated LCs to break tolerance and stimulate effective CTLs against this broadly-expressed, self-differentiation tumor Ag. Successful completion ofthese aims will fundamentally alter management of graft-host interactions in the context of allogeneic HSCT by controlling immune responses regulated by distinct DC subsets from their onset. These studies will also have broader public health potential to improve management of immune suppression and prevention of graft rejection in solid organ transplantation.